There are growing demands to fabricate polymeric thin films with vertically aligned sub-nanometer channels for applications including carbon capture, gas separation, water desalination, batteries, fuel cell membranes, and solar-fuel conversion. Obtaining molecular level control over the pore size, shape and surface chemistry is a critical bottleneck, and has been investigated across many disciplines. Inorganic or hybrid porous materials, like zeolites and metal organic frameworks, where pore size can be readily tailored from a few tenths to several nanometers, have achieved some success along these lines. However, it remains a challenge to orient the channels over macroscopic distances and to process these materials on flexible substrates.
Polymers, on the other hand, are readily processed at low cost as thin films or multilayered laminates with little, if any, constraints on chemical structure and thickness of the individual layers. Hypercrosslinked polymer networks can provide pores as small as a few tenths of nanometers, but the size distribution of the pores is broad and the spatial arrangement is random which impedes selective transport. Alternatively, by using block copolymers (BCPs), it is now routine to produce nanoporous thin films with nearly monodisperse aligned pores as small as 3 nm. However, even the smallest pore size obtained in a BCP thin film is too large for the selective molecular transport required in advanced applications including the aforementioned areas.
Composite films have been fabricated using preformed nanotubes, like carbon nanotubes (CNTs). However, there are no effective means to orient preformed nanotubes normal to the surface over macroscopic distances. Thus, vertically aligned CNT forests have been grown on substrates, yet it remains non-trivial to backfill with matrix materials between CNTs to achieve lift-off from the underlying substrate generating through channels. CNTs prepared via this route are still subject to some degree of heterogeneity in their pore size distribution, which may be a further limiting factor in their implementation as selective membranes. Interior modification of preformed nanotubes to enhance selectively also represents a significant hurdle.
In contrast to CNTs, sub-nanometer tubular materials with precise control over the pore size, shape and surface chemistry can be produced by assembling organic motifs, like cyclic peptides, dendrimers, DNA, surfactants and rosettes. Unlike preformed-nanotubes such as CNTs, the formation of nanotubes based on organic subunits is reversible, as they are governed by specific inteimolecular interactions, like hydrogen bonding or electrostatic interactions. A directed, synergistic co-assembly of nanotube subunits and BCPs may allow one to direct the formation of nanotubes within the nanoscopic domains established by the BCPs so as to manipulate the spatial organization and macroscopic orientation of nanotubes. This process takes full advantage of nanoscopic assembly of BCPs and the reversibility of organic nanotube growth and is compatible with existing technologies for thin film fabrication. It completely eliminates the need to prepare nanotubes of uniform length, since the number of subunits comprising the nanotube can easily be varied so that the nanotube length is tailored to be equal to the film thickness, thus producing porous films with nanotubes that span the entire film. This new strategy also circumvents all impediments associated with aligning and organizing high aspect ratio nano-objects normal to the surface. Surprisingly, the present invention meets this and other needs.